Running a pipe segment into or out of a wellbore during the drilling, completion, or production phases of oil and gas well operations often requires adding or removing the pipe segment when the wellbore is under pressure. Typically this wellbore pressure is retained by pressure control equipment having resilient internal seals that seal the wellbore opening around the pipe string. Running pipe into the wellbore under pressure is called snubbing. During snubbing significant downward force on the pipe is often required in order to urge the pipe through the internal seals of the pressure control equipment used to retain the wellbore pressure.
Snubbing typically requires special equipment called snubbing units. A typical snubbing unit has a traveling frame that supports a traveling pipe gripping device such as a traveling slip-type spider and that moves upward and downward between upper and lower stationary frames. The upper stationary frame, called a work basket, has railing and decking and serves as a platform for workers. The lower stationary frame is positioned around the wellbore pressure control equipment such as a stack of blow out preventers (BOP stack) with retractable internal rams that seal the opening into the wellbore. The lower stationary frame has a stationary spider used to hold and support the wellbore pipe string.
The traveling frame is attached to and supported upon the extendable and retractable pistons of large hydraulic jacks or cylinders mounted between the lower stationary frame and the traveling frame. Extension and retraction of the pistons of the hydraulic cylinders moves the traveling frame vertically upward and downward with respect to the lower stationary frame. This upward and downward movement allows the traveling frame, with the traveling spider, to be extended upward through the opening in the workbasket and downward to a position approaching the lower stationary frame and stationary pipe gripping device. The traveling frame and hydraulic cylinder combination is sometimes called a snubbing jack and the cylinder of the snubbing jack can be sized as desired to produce a desired snubbing load or force.
When a new pipe segment is added to a wellbore pipe string during snubbing, the lower stationary frame is positioned around the pressure control equipment of the wellbore with the stationary spider in vertical alignment with the wellbore and with the sealed opening of the pressure control equipment. The wellbore pipe string is supported by the stationary spider on the stationary frame. The traveling frame is extended upward from the lower stationary frame by extension of the pistons of the hydraulic cylinders to extend into the opening of the workbasket.
A new pipe segment is then positioned to extend vertically downward from the workbasket, gripped by the traveling spider of the traveling frame, and held in position over the wellbore pipe string. The pistons of the hydraulic cylinders supporting the traveling frame are then retracted to lower the traveling frame and the new pipe segment toward the wellbore pipe string for connection to the wellbore pipe string. A top drive, rotary or similar rotating equipment is used to rotate the new pipe segment to connect it to the wellbore pipe string. The tubular pistons are then further retracted to push or snub the new pipe segment through the internal seals of the pressure control equipment to advance the pipe string into the wellbore. This process is reversed when a pipe is removed from the wellbore.
Because snubbing is done under wellbore pressure, the weight of pipe string in the wellbore may be less than the force generated on the pipe string by the wellbore pressure. In that situation the wellbore pressure may push the wellbore pipe string upward out of the wellbore. To prevent the pipe string from being ejected from the wellbore due to the wellbore pressure, the traveling and stationary frames and their associated pipe gripping devices must be positioned to maintain firm control of both the wellbore pipe string and the pipe segment being added.
One problem associated with typical snubbing units is that the pipe being snubbed is unsupported along its length when the pistons of the hydraulic cylinders are retracted to force the pipe string downward into the wellbore against the upward wellbore pressure. The compressive forces placed on the unsupported pipe during snubbing increases the risk that the pipe will buckle as it is pushed through the internal seals of the pressure control equipment. To reduce the risk of pipe buckling as the traveling frame is lowered to snub the pipe string into the wellbore, only relatively short segments of pipe are inserted on each cycle of the traveling frame. This increase the time and costs associated with a typical snubbing operation.